1. Field of the Invention
The present invention relates to a wire electric discharge machine.
2. Description of the Related Art
A wire electric discharge machine machines a workpiece into a desired shape by changing the relative position between the wire electrode and the workpiece while causing discharge by applying a voltage to the machining gap between the wire electrode and the workpiece. The machining results of the workpiece depend on its material, thickness, and so on, and an accuracy of several micrometers may be required in high accuracy machining.
When high accuracy machining results are required, after completion of the first machining, machining into the same machining shape is performed a plurality of times while changing the offset. The number of machining processes is determined depending on the required machining results such as the surface roughness and geometric accuracy of a machined product. Here, it is assumed that the first machining process is referred to as rough machining and the second and subsequent machining processes are referred to as finish machining.
First, rough machining will be described.
An exemplary pulse generation circuit of the machining power supply of a wire electric discharge machine is shown in FIG. 9. Reference numeral 1 indicates the machining power supply. An AC pulse voltage is applied to the gap between a wire electrode 10 and a workpiece 11 by alternately turning on and off switching elements 3, 4, 5, and 6 present in a secondary power supply 2. After the pulse voltage causes a dielectric breakdown between the wire electrode 10 and the workpiece 11, a primary power supply 7 applies a current pulse. Reference numerals 8 and 9 indicate switching power supplies.
Next, finish machining will be described.
Since finish machining does not require much machining energy, only the secondary power supply 2 is used for the machining. An AC pulse voltage is applied to the gap between the wire electrode 10 and the workpiece 11 to perform machining. The voltage value, pulse width, and frequency of the pulse voltage to be applied are determined depending on required machining results such as the surface roughness and geometric accuracy of a machined product.
In the conventional method described above, when finish machining (particularly, the second machining) is performed, the magnitude of machining energy becomes a problem. At the time when the first machining is completed, the surface roughness and geometric accuracy of the machined product is not highly precise. On the other hand, since an AC pulse voltage is applied only by the secondary power supply 2 in the second machining, the machining energy may become insufficient.
Although an approach that performs the second machining as in the first machining may be considered, even if an attempt is made to reduce the machining energy using the pulse generation method of the first machining, stable machining cannot be done. This is because there are variations in the response speed of MOS FETs used as switching elements and variations in elements of the control circuit for turning on and off the MOS FETs. Variations in elements of the control circuit may prevent on-off instructions from being transmitted as intended. Even if on-off instructions are stable, since switching elements also have variations in their responses to instructions, the on-off time may vary even for the same instruction for each of the switching elements.
Accordingly, the machining energy may become insufficient in the second machining such as finish machining and there is no method for supplying appropriate machining energy stably.
As described above, the wire electric discharge machine machines the workpiece into a desired shape by changing the relative position between the wire electrode and the workpiece while causing discharge by applying a voltage to the machining gap between the wire electrode and the workpiece. The machining results of the workpiece depend on its material, thickness, and so on, and an accuracy of several micrometers may be required in high accuracy machining.
When the wire electric discharge machine performs machining into a desired machining shape, if a high accuracy of several micrometers is not required, machining into the desired machining shape is performed only once to obtain machining results. In contrast, if high accuracy machining results are required, after the first machining is completed, machining into the same machining shape is performed a plurality of times while the offset is changed. The number of machining processes is generally determined depending on the required machining results such as the surface roughness and geometric accuracy of a machined product.
Here, it is assumed that the first machining process is referred to as rough machining and the second and subsequent machining processes are referred to as finish machining.
First, rough machining will be described.
An exemplary pulse generation circuit of the machining power supply of the wire electric discharge machine is shown in FIG. 9. The switching elements present in the secondary power supply 2 are turned on and off alternately to apply an AC pulse voltage to the gap between the wire electrode 10 and the workpiece. After the pulse voltage causes a dielectric breakdown between the wire electrode 10 and the workpiece 11, the primary power supply 7 applies a current pulse.
Rough machining requires much machining energy since it machines a workpiece that has not been machined yet. Strictly speaking, the machining can be performed only by the secondary power supply 2. However, as described in Japanese Patent Application Laid-Open No. 2004-195562, the structure including the primary power supply 7 and the secondary power supply 2 are generally used to improve the machining efficiency.
Next, finish machining will be described.
Since finish machining does not require much machining energy, only the secondary power supply 2 is used for the machining. An AC pulse voltage is applied to the gap between the wire electrode 10 and the workpiece 11 to perform machining. The voltage value, pulse width, and frequency of the pulse voltage to be applied are determined depending on the required machining results such as the surface roughness and geometric accuracy of a machined product.
When high accuracy machining is performed through a plurality of finish machining processes, the machining energy is generally made smaller with each subsequent machining. The second machining such as finish machining is performed with less machining energy than in the first machining such as rough machining and the third machining such as finish machining is performed with less machining energy than in the second machining. A technique for high accuracy finish machining is disclosed in Japanese Patent Application Laid-Open No. 2010-194693.
A problem with a prior art technique when the second machining such as finish machining is performed after the first machining such as rough machining is finished will be described. As preparation for the description, the switching elements for generating pulses in FIG. 9 will be described.
When the secondary power supply 2 applies an AC pulse voltage in rough machining and finish machining, switching with a high frequency in the range from hundreds of kilohertz to several megahertz is performed. In the switching with a high frequency, semiconductor switching elements such as MOS FETs are generally used.
In rough machining, the pulse input timing of the primary power supply 7 depends on the situation of a dielectric breakdown between the wire electrode 10 and the workpiece 11 caused by the AC pulse voltage of the secondary power supply 2. The pulse width for each pulse current of the primary power supply 7 is small (for example, several microseconds). Generally, semiconductor switching elements such as MOS FETs are also used for switching of the primary power supply 7.
A problem when the second machining such as finish machining is performed is the magnitude of machining energy. At the time when the first machining such as rough machining is completed, the accuracy of the surface roughness and geometric accuracy of the machined product is not highly precise. On the other hand, since an AC pulse voltage is applied only by the secondary power supply 2 in the second machining such as finish machining, the machining energy may become insufficient.
On the other hand, an approach that performs the second machining such as finish machining as in the first machining by reducing the machining energy may be considered. Even if an attempt is made to reduce the machining energy using the pulse generation method of the first machining, stable machining cannot be done. This is because there are variations in the response speed of MOS FETs used as switching elements and variations in elements of the control circuit for turning on and off the MOS FETs.
Generally, elements of the circuit each have variations in their characteristic values. Variations in elements of the control circuit may prevent on-off instructions from being transmitted as intended. In this case, the on-off time of switching elements of the primary power supply and the magnitude of a current pulse are changed.
Even if on-off instructions are stable, since the switching elements also have variations in their responses to the instructions, the on-off time may vary even for the same instruction for each of the switching elements. Accordingly, the magnitude of a current pulse is changed.
For the reasons described above, even if the pulse generation method for the first machining such as rough machining is used for the second machining such as finish machining, a stable pulse current cannot be supplied, thereby preventing stable machining. As described above, in the prior art technique, the machining energy may become insufficient when the second machining such as finish machining is performed after completion of the first machining such as rough machining and there is no method for supplying appropriate machining energy.
Machining using capacitor pulses in which a pulse current is stored in a capacitor is disclosed in, for example, Japanese Patent Application Laid-Open No. 2005-246551, but sufficient machining efficiency cannot be obtained because the capacitor is connected to only one polarity.